Electric fuse

ABSTRACT

An electric fuse includes a dielectric casing having an explosion chamber and an open end communicating with the chamber, a pair of contacts disposed in the casing, an electric fuse element disposed in the chamber and connected in series with the contacts, and a metallic cap covering the open end. The cap may be apertured and includes a transparent dielectric sheet between the cap and casing to provide a window. The dielectric casing has a plurality of closely spaced alternating longitudinal ridges and grooves along the outer side adjacent the open end extending from the open end toward the base of the casing. The metallic cap tightly covers the open end and is in contact with the ridged side of the casing and includes a rim which is bent inwardly to grip the dielectric casing below the end of the ridges. Upon explosion of the fuse element, gas formed in the explosion chamber temporarily lifts the cap and sheet and escapes past the open end under the cap, with the space between the cap and the grooves providing a path for the gas in the direction away from the open end toward the base of the casing. The improved venting structure permits use of moldable thermosetting plastic materials for the casing.

United States Patent [191 Aug. 27, 1974 Griffin ELECTRIC FUSE [75] Inventor: Richard H. Griffin, Riverside, R1.

[73] Assignee: International Telephone and Telegraph Corporation, Nutley, NJ. [22] Filed: July 27, 1973 [21] Appl. No.: 383,221

[52] US. Cl 337/272, 337/203, 337/250 [51] Int. Cl. H0lh 85/14 [58] Field of Search 337/203, 249, 250, 281,

[56] References Cited UNITED STATES PATENTS 1,001,694 8/1911 Sargent 337/272 1,621,349 3/1927 Brown 337/272 2,017,489 10/1935 Glowacki 337/272 X 2,034,366 3/1936 Wynn 337/272 X 2,065,338 12/1936 Ludwig 337/272 2,704,796 3/1955 Manga 337/272 X Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm--.lohn T. OHalloran; Menotti .l. Lombardi, Jr.; Edward Goldberg [57] ABSTRACT An electric fuse includes a dielectric casing having an explosion chamber and an open end communicating with the chamber, a pair of contacts disposed in the casing, an electric fuse element disposed in the chamber and connected in series with the contacts, and a metallic cap covering the open end. The cap may be apertured and includes a transparent dielectric sheet between the cap and casing to provide a window. The dielectric casing has a plurality of closely spaced alternating longitudinal ridges and grooves along the outer side adjacent the open end extending from the open end toward the base of the casing. The metallic cap tightly covers the open end and is in contact with the ridged side of the casing and includes a rim which is bent inwardly to grip the dielectric casing below the end of the ridges. Upon explosion of the fuse element, gas formed in the explosion chamber temporarily lifts the cap and sheet and escapes past the open end under the cap,'with the space between the cap and the grooves providing a path for the gas in the direction away from the open end toward the base of the casing. The improved venting structure permits use of moldable thermosetting plastic materials for the casing.

6 Claims, 3 Drawing Figures ELECTRIC FUSE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electric fuses and more particularly to improved venting means provided on the dielectric casing.

2. Description of the Prior Art In general, an electric fuse, such as the type found in private homes, includes a dielectric casing of porcelain or ceramic having an explosion chamber, an open end communicating with the chamber and a pair of electric contact elements disposed in the casing. The fuse element is disposed in the chamber and is connected in series with the contact elements, with the open end enclosed by a suitable covering means, The covering means is usually a metallic cap and a'transparent dielectric sheet, typically of mica, inserted between the open end of the dielectric casing and the metallic cap. The cap is centrally apertured to permit visual inspection of the fuse element.

It is essential, for safety reasons, that the fuse body be designed to withstand the high momentary heat and pressure created by the hot gas formed in the explosion chamber upon explosion or burnout of the fuse element, particularly to prevent ignition of any combustible material by the hot gas. Typically, according to the prior art, rather complex passages are provided in the covering means which communicate with the explosion chamber. The hot gas formed by the explosion of the fuse element enters the passages and is permitted to cool somewhat before escaping from the casing through vents provided therefrom. The passages are generally positioned over the open end of the dielectric casingwithin the metallic cap and the vents are provided on the top or along the side of the cap such that the gas is usually allowed to escape toward the face or side of the cap, and then outside. This arrangement may expel relatively hot gas to objects located around the fuse that can be readily combustible.

Two prior art types of fuses are shown in US. Pat. No. 804,741 issued Nov. 14, 1905 and US. Pat. No. 2,068,584 issued Jan. 19, 1937. The first fuse device has four widely spaced passages extending along the sides of the fuse body from an open end and has a cap supported above the end on struts. An apertured disc covers the open end and includes a telltale indicator in the opening. When the fuse element blows, the telltale is dislodged and gas escapes through the cap and passages. The second device has a cap fitting closely over the end and having knurled sides and of flange which grips teeth in a lateral peripheral edge around the fuse body. The teeth provide gas venting channels around the edge when gas pressure causes the flange to flex, with the gas passing under the cap and along the sides through an annular space to the channels formed between the cap and teeth under the edge. The fuse bodies in these devices are usually made of porcelain materials which have high strength but are costly and difficult to manufacture.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an electric fuse having a simplified structure.

It is anotherobject of the present invention to provide an improved venting means in an electric fuse to minimize fire hazard and permit use of a moldable thermosetting plastic body.

These and other objects of the present invention are attained, in accordance with a preferred embodiment of the present invention, by providing a dielectric casing having a plurality of closely spaced alternating longitudinal grooves and ridges along the outer side surface adjacent the open end and extending from the open end toward the base of the casing. The cap tightly covers the open end and contacts the ridged sides of the casing and includes a rim which is bent inwardly to grip the casing. The hot gas formed in the explosion chamber upon explosion of the fuse element passes over the end under the cap and escapes through the vents formed by the grooves along the sides in the direction away from the open end toward the base of the casing. This is a significantly safer way of venting the hot gas than that of previously known type of fuses since the hot gas is now allowed to escape toward the base of the fuse which is surrounded by a fireproof socket assembly. The force of the exploding gas temporarily lifts the cap through a minute distance above the open end of the casing and allows the gas to escape therethrough. The limited opening at the end has the effect of gradually releasing the relatively hot gas and reducing the chamber pressure slowly. This permits use of a thermosetting plastic material which can be molded more readily into a desired casing shape than the ceramic materials typically used in the prior art fuse assembly.

It is a further feature of the present invention that one of the conductive leads for the fuse is in the form of a curved loop or ring extending around the outer side of the dielectric casing and having a spring portion to apply a positive pressure for securely engaging an electric contact of a socket into which the fuse may be inserted.

These and other objects and advantages of the present invention may be more fully understood from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a three dimensional view of an electric fuse in accordance with the present invention;

FIG. 2 shows a cross-sectional side view of the electric fuse; and

FIG. 3 is a cross-sectional bottom view of the electric fuse along line AA of FIG. 2 showing a plurality of ridges and grooves which provides venting for the hot gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, an electric fuse in accordance with the present invention includes a dielectric casing 10 preferably of a suitable thermosetting plastic material such as phenolic, urea or melamine, a centrally apertured metallic cap 11 covering an open end of the casing 10, and a transparent dielectric sheet 12, such as of mica, interposed between the casing 10 and the metallic cap 11. As shown in FIG. 2, the casing has an explosion chamber 13 which houses a fuse element 14 made of a suitable metal such as zinc. One end of the fuse element is connected to a conductive contact 15 disposed through the bottom of the casing for engagement with a corresponding contact of an electrical socket into which the fuse is inserted. The other contact 16 is a commonly known S-type contact, one end of which is connected to the fuse and the other end of which extends through a side aperture 17 in a shoulder l8 and wraps around a substantial portion of the outer circumference of the casing. A spring portion 19 is provided along the external contact so that when the fuse is inserted in a socket the bias of that spring portion assures a positive connection to securely engage the contact of the socket. Typically the outside surface of the dielectric casing is threaded along the lower portion 20 so that the fuse may be screwed into the socket member of a power source (not shown).

In accordance with the present invention, the outer surface along the side of the open end of the dielectric casing 10 is contoured to having a plurality of closely spaced alternating longitudinal ridges 21 and grooves 22 running from the edge of the open end toward the base of the dielectric casing 10. Other variations may include knurled or notched surfaces. The top edge 23 of the open end of the dielectric casing 10 is flat as is the dielectric sheet 12 so that, when assembled with the cap 11, a tight fit is provided which normally closes off the explosion chamber from the outside. The metallic cap member 11 has a circular aperture 24 to permit vi sual inspection of the fuse element 14. The side of the cap surrounds and contacts the ridges 21 along the side of the casing and has a rim 25 that is bent inwardly to grip the dielectric casing 10 below the end of the ridges. The grip may be further strengthened by forming sawtooth or other similar indentations 26 at the lower end of the ridges and grooves and dimensioning the casing 10 to have a narrower outer diameter thereabout, with the rim being bent inwardly over the indentations.

When the fuse element 14 opens, upon the occurrence of overloading current such as caused by a short circuit, hot gas is formed in the explosion chamber at a relatively high temperatures and pressures. The high pressure temporarily lifts the dielectric sheet 12 and provides a relatively small space between the top edge 23 of the casing and the dielectric sheet. This not only permits a gradual escape of the gas formed in the explosion chamber but also relieves some amount of pressure built up in the chamber. The gas is allowed to escape over the edge and then passes through the escape paths formed by the grooves under the cap in a direction away from the open end toward the base of the casing. The possible fire hazard from the high temperature gas escaping from the front or side of the fuse is thus minimized by directing the hot gas through the vents along the side of the casing while permitting substantial cooling before the gas passes out toward the base of the fuse which is surrounded by the fireproof socket assembly. The particular structure and operation of the fuse assembly are designed to meet the safety requirements of standard Underwriter Laboratories tests and specifications.

The momentary high pressure exerted by the gas when the fuse explodes is reduced by the assembly of the cap and the intermediate sheet over the end of the casing with a sufficiently tight fit such that the cap and transparent sheet 12 can be lifted temporarily. The pressure relief provided to the exploding gas in this manner makes it possible to use a more readily moldable thermosetting plastic material for the casing in place of the usual ceramic material which has greater strength against explosion but is more costly and difficult to manufacture. The present invention therefore provides an improved simplified fuse structure having a greater degree of safety.

What is claimed is:

1. An electric fuse comprising:

a dielectric casing having an explosion chamber therein and an open end communicating with said chamber; electric contacts disposed in said casing; an electric fuse element connected in series with said electric contacts and positioned in said chamber;

means for tightly covering said open end to normally close said chamber and adapted to be temporarily lifted to open said chamber upon explosion of said fuse element, a portion of said covering means extending over said end and along and in contact with the outer side of said casing; and

venting means including a plurality of closely spaced alternating longitudinal grooves and ridges in the. surface of said dielectric casing surrounding said outer side and extending from said open end toward the base of said casing for providing an escape path for the gas formed in the chamber upon said explosion of said fuse element, said covering means being in contact with said ridges extending along said outer side from said open end, said path including spaces between said grooves and said portion of said covering means along said outer side directing the gas away from said open end toward said base.

2. The electric fuse of claim 1, wherein said casing includes a narrow outer diameter portion spaced from said open end, and said plurality of grooves and ridges in said surface extend from the edge of said open end to said narrow outer diameter portion.

3. The electric fuse of claim 2, wherein said covering means includes a metallic cap and a sheet of transparent dielectric material interposed between said metallic cap and the open end of said casing, said cap having a centrally positioned aperture, a portion of said metallic cap extending along the outer side of said casing in contact with said plurality of ridges between said open end and narrow portion of said casing and having a rim bent inwardly over said narrow portion to grip said casing below the end of said ridges.

4. The electric fuse of claim 3, wherein said electric contacts include a pair of conductive elements, one of said pair being disposed through the bottom of said casing and the other of said pair being disposed through a side of said casing below said cap and extending around the outer surface of said casing and having a spring portion adapted to establish a firm contact against an external conductive terminal to which said fuse is connected. 5. The electric fuse of claim 3, wherein said cap and sheet have a relatively tight fit over said open end and are adapted to be lifted temporarily to provide a space between the top edge of said casing and said sheet to permit the escape of said gas along said side through said grooves.

6. The electric fuse of claim 5, wherein said dielectric casing is made of a moldable thermosetting plastic material. 

1. An electric fuse comprising: a dielectric casing having an explosion chamber therein and an open end communicating with said chamber; electric contacts disposed in said casing; an electric fuse element connected in series with said electric contacts and positioned in said chamber; means for tightly covering said open end to normally close said chamber and adapted to be temporarily lifted to open said chamber upon explosion of said fuse element, a portion of said covering means extending over said end and along and in contact with the outer side of said casing; and venting means including a plurality of closely spaced alternating longitudinal grooves and ridges in the surface of said dielectric casing surrounding said outer side and extending from said open end toward the base of said casing for providing an escape path for the gas formed in the chamber upon said explosion of said fuse element, said covering means being in contact with said ridges extending along said outer side from said open end, said path including spaces between said grooves and said portion of said covering means along said outer side directing the gas away from said open end toward said base.
 2. The electric fuse of claim 1, wherein said casing includes a narrow outer diameter portion spaced from said open end, and said plurality of grooves and ridges in said surface extend from the edge of said open end to said narrow outer diameter portion.
 3. The electric fuse of claim 2, wherein said covering means includes a metallic cap and a sheet of transparent dielectric material interposed between said metallic cap and the open end of said casing, said cap having a centrally positioned aperture, a portion of said metallic cap extending along the outer side of said casing in contact with said plurality of ridges between said open end and narrow portion of said casing and having a rim bent inwardly over said narrow portion to grip said casing below the end of said ridges.
 4. The electric fuse of claim 3, wherein said electric contacts include a pair of conductive elements, one of said pair being disposed through the bottom of said casing and the other of said pair being disposed through a side of said casing below said cap and extending around the outer surface of said casing and having a spring portion adapted to establish a firm contact against an external conductive terminal to which said fuse is connected.
 5. The electric fuse of claim 3, wherein said cap and sheet have a relatively tight fit over said open end and are adapted to be lifted temporarily to provide a space between the top edge of said casing and said sheet to permit the escape of said gas along said side through said grooves.
 6. The electric fuse of claim 5, wherein said dielectric casing is made of a moldable thermosetting plastic material. 